1. Field of the Invention
The present invention relates to an SSB (single side-band) communication system. More specifically, this invention relates to apparatus and methods for a demodulating process of SSB signals where a frequency detector and amplitude limiter can be employed.
It is desirable when communicating with radios that degradations caused by multi-path fading are completely removed. Therefore, modulation methods which can counter the effects of fading must be used. The immediate solution would be to use some form of angle modulation such as FM (frequency modulation) or PM (phase modulation). However, the necessary FM transmission bandwidth of B is approximately twice the sum of the peak frequency deviation of .DELTA.F and the highest modulating frequency of f.sub.m, that is, B=2(.DELTA.F+f.sub.m). This relation is called Carson's rule, and indicates that the FM transmission bandwidth is much wider than the information itself. Even if the frequency deviation is limited to zero, the necessary FM bandwidth is twice as large as the information bandwidth, namely, 2f.sub.m.
Using SSB modulation, voice signals can be inherently transmitted in a bandwidth comparable to the information bandwidth. Therefore, an SSB modulation method is very useful from the view-point of a considerable saving of the bandwidth. However, is SSB, the information is contained in the signal envelope. For the purpose of maintaining the high quality of demodulated voice signals, an AGC (automatic gain control) circuit fast enought to follow fading must be provided. Also, the carrier must be regenerated in order to demodulate the SSB signal. Carrier recovery is a cumbersome operation for mobile radios.
2. Description of the Prior Art
There are a lot of transmitters which employ an SSB modulation method, however, SSB signals cannot be demodulate without using a carrier regenerated in a receiver. Also SSB signals cannot remove amplitude variations with an amplitude limiter. In the technical paper entitled "Information in the zero crossings of bandpass signals", Bell System Technical Journal, vol. 56, no. 4, p.p. 487-510, April 1977, the author of the paper, B. F. Logan pointed out that the signal can be reconstructed after amplitude limiting if the signal is expressed by only real simple zeros and that a full-carrier SSB signal belongs to the signal expressed by only real simple zeros.